1. Field of the Invention
The present invention relates to an active matrix type display apparatus including luminescent elements such as EL (Electro-luminescence) elements or LED (Light Emitting Diode) elements which emit light by driving current flowing in thin films of organic semiconductors or the like, and also including thin film transistors (hereinafter TFT""s) to control the emitting operation of these luminescent elements. More particularly, the present invention relates to a technique of driving each element formed in this type of display apparatus.
2. Description of the Related Art
Active matrix type display apparatuses incorporating luminescent elements of a current controlling type, such as EL elements and LED elements, have been proposed. Since any luminescent element employed in this type of display apparatus emits by itself, there are advantages in using no back-light and in having a minimal dependence on the viewing angle and the like.
FIG. 31 is a block diagram illustrating an active matrix type display apparatus incorporating organic thin film EL-elements of an electric charge filling type, as an example of these types of display apparatuses. In the display apparatus 1A shown in this figure, a plurality of scanning lines xe2x80x9cgatexe2x80x9d, a plurality of data lines xe2x80x9csigxe2x80x9d extending in a direction that intersects the direction in which the scanning line xe2x80x9cgatexe2x80x9d extend, a plurality of common power supply lines xe2x80x9ccomxe2x80x9d extending parallel to the data lines xe2x80x9csigxe2x80x9d, and a plurality of pixels 7 located at the intersections of the data lines xe2x80x9csigxe2x80x9d and the scanning lines xe2x80x9cgatexe2x80x9d which are formed on a transparent substrate.
Each pixel 7 comprises a first TFT 20 in which a scanning signal is supplied to the gate electrode (a first gate electrode) through the scanning gate, a holding capacitor xe2x80x9ccapxe2x80x9d which holds an image signal supplied from the data line xe2x80x9csigxe2x80x9d via the first TFT 20, a second TFT 30 in which the image signal held by the holding capacitor xe2x80x9ccapxe2x80x9d is supplied to the gate electrode (a second gate electrode), and an luminescent element 40 (indicated as a resistor) into which the driving current flows from the common power supply line xe2x80x9ccomxe2x80x9d when the element 40 is electrically connected to the common power supply line xe2x80x9ccomxe2x80x9d through the second TFT 30.
In the above display apparatus 1A, both the first TFT 20 and the second TFT 30 are conventionally formed, as with an N channel type TFT or a P channel type TFT, as shown in an equivalent circuit diagram of FIG. 32, from the viewpoint of simplifying the production process, for example, in the case of an N channel type. Taking the N channel type as an example, as shown in FIGS. 33(A) and (B), when the high potential image signal xe2x80x9cdataxe2x80x9d is written into the holding capacitor xe2x80x9ccapxe2x80x9d from the data line xe2x80x9csigxe2x80x9d, while the scanning signal xe2x80x9cSgatexe2x80x9d supplied through the scanning line xe2x80x9cgatexe2x80x9d has become higher in potential to turn the first TFT 20 xe2x80x9conxe2x80x9d, the second TFT 30 is held in the xe2x80x9conxe2x80x9d state. Consequently, in the luminescent element 40, the driving current keeps flowing from a pixel electrode 41 to an opposite electrode xe2x80x9copxe2x80x9d in the direction indicated by the arrow xe2x80x9cExe2x80x9d and consequently, the luminescent element 40 keeps emitting (the xe2x80x9conxe2x80x9d state). On the other hand, when the image signal xe2x80x9cdataxe2x80x9d, which is lower than the intermediate between the potential of the common power supply line xe2x80x9ccomxe2x80x9d and the potential of the opposite electrode xe2x80x9copxe2x80x9d, is written into the holding capacitor xe2x80x9ccapxe2x80x9d from the data line xe2x80x9csigxe2x80x9d, while the scanning signal xe2x80x9cSgatexe2x80x9d supplied through the scanning line xe2x80x9cgatexe2x80x9d has become higher in its potential to turn the first TFT 20 xe2x80x9conxe2x80x9d, the second TFT 30 is turned xe2x80x9coffxe2x80x9d and consequently, the luminescent element 40 is turned xe2x80x9coffxe2x80x9d (the xe2x80x9coffxe2x80x9d state).
In the above display apparatus 1A, a semiconductor thin film, an insulating thin film, an electrode, etc., which constitute each element, are formed by thin films deposited on the substrate. Considering the heat resistance of the substrate, a low-temperature process is often used to form the thin films. Therefore the quality of the thin film is poor, as is evidenced by the frequent defects caused by a physical-property difference between a thin film and a bulk, which result in problems such as an electrical breakdown, and wherein time-degradation is apt to arise in the TFT and similar devices.
In the case of a liquid crystal display apparatus incorporating liquid crystals as light modulation elements, although it also uses the thin films, time-degradation can be suppressed not only in the liquid crystal but also in the TFT, because the light modulation element is driven by AC power. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, time-degradation is more often encountered in the TFT than in the liquid crystal display apparatus insofar as the apparatus is essentially driven by D.C. power. Although improvements have been made in the structure of the TFT and the process techniques in the display apparatus 1A, incorporating luminescent elements of the current controlling type, they do not yet seem to be improved enough.
In the case of incorporating the liquid crystals as the light modulation elements, the power consumption is small because the light modulation element is controlled by the voltage which causes the current flow in each element to be only momentary. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, a constant driving current is required to keep the luminescent element xe2x80x9conxe2x80x9d, and this results in high power consumption and the risk of the frequent occurrence of electrical breakdown and time-degradation.
Further, in the liquid crystal display apparatus, the liquid crystal can be AC-driven by one TFT per one pixel. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, the luminescent element 40 is DC-driven by two TFTs 20, 30 per one pixel. This raises the driving voltage, and exacerbates the aforementioned problems, such as electrical breakdown and time-degradation. For example, as shown in FIG. 33(A), the gate voltage xe2x80x9cVgswxe2x80x9d of the first TFT, when selecting a pixel, corresponds to the potential difference between the potential equals to the higher potential of the scanning signal xe2x80x9cSgatexe2x80x9d and the potential of the potential-holding electrode xe2x80x9cstxe2x80x9d (the potential of the holding capacitor xe2x80x9ccapxe2x80x9d or the potential of the gate electrode of the second TFT 30). Therefore when the potential of the potential-holding electrode xe2x80x9cstxe2x80x9d and, hence, the gate voltage xe2x80x9cVgcurxe2x80x9d of the second TFT 30 are raised to make the luminescent element 40 emit in a high luminance, the gate voltage xe2x80x9cVgswxe2x80x9d of the first TFT 20 is lowered correspondingly. Therefore, the greater amplitude of the scanning signal xe2x80x9cSgatexe2x80x9d has to be employed, requiring the higher driving voltage in the display apparatus 1A. Besides, in the aforementioned display apparatus 1A, since when the luminescent element 40 is xe2x80x9coffxe2x80x9d, the potential of the image signal xe2x80x9cdataxe2x80x9d is made lower than the intermediate potential between the potential of the common power supply line xe2x80x9ccomxe2x80x9d and the potential of the opposite electrode xe2x80x9copxe2x80x9d in order to turn the second TFT 30 xe2x80x9coffxe2x80x9d, there is another problem of increased amplitude of the image signal xe2x80x9cdataxe2x80x9d. Accordingly, in this display apparatus 1A, special consideration for the power consumption and the withstanding of voltage of the TFT, etc. is needed compared to the liquid crystal display apparatus. However, the conventional display apparatus 1A has not been provided with the sufficient consideration of these factors.
Accordingly, an object of the present invention is to provide a display apparatus, which improves display image quality as well as suppresses power consumption, electric breakdown and deterioration with time by reducing the driving voltage, relying upon a driving method which takes into account the conduction types of TFTs used for controlling emission operations of the current-driven light-luminescent elements so as to reduce the driving voltage, which improves both the display image quality and characteristics such as power consumption, breakdown and time-degradation.
To accomplish the aforementioned object, the present invention in proposes a display apparatus comprising, arranged on a substrate, a plurality of scanning lines, a plurality of data lines intersecting the scanning lines, a plurality of common power supply lines, a plurality of pixels formed by the scanning lines and the data lines in a matrix form, each of said pixels comprising a first TFT having a first gate electrode which is supplied with a scanning signal through the scanning line, a holding capacitor which holds an image signal supplied by the data line though the first TFT, a second TFT having a second gate electrode which is supplied with the image signal held by the holding capacitor, an emitting thin film, which emits light due to the driving current which flows between the pixel electrode and an opposite electrode, which is opposed to the pixel electrode with the emitting thin film provided therebetween, when the pixel electrode is electrically connected to the common power supply line through the second TFT, wherein the potential of the common power supply line is set at a lower level than that of the opposite electrode when the second TFT is of an N channel type.
In the display apparatus according to the present invention, since the gate voltage of the second TFT at xe2x80x9conxe2x80x9d state corresponds to the difference between the potential of gate electrode (the potential of the image signal) and one of the potential of the common power supply line and the pixel electrode, the gate voltage of the second TFT is arranged so as to correspond to the potential difference between the common power supply line and the potential-holding electrode by optimizing relative potential values between the common power supply line and the opposite electrode of the luminescent element according to the conduction type of the second TFT. For example, when the second TFT is of an N channel type, the potential of the common power supply line is set at a lower level than that of the opposite electrode of the luminescent element. Since this potential of the common power supply line can be set low enough different from the potential of the pixel electrode, the large xe2x80x9conxe2x80x9d current in the second TFT can be obtained to get a high luminance display. If the higher potential can be obtained in the second TFT when the pixel is turned xe2x80x9conxe2x80x9d, the potential of the image signal can be lowered to reduce its amplitude, and this results in a reduction of the driving voltage in the display apparatus. Therefore, there are advantages in reducing the power consumption, and simultaneously the problem of withstanding voltage, which concerns each element formed by a thin film, is not encountered.
In accordance with the present invention, if the second TFT is of an N channel type, it is preferable that the potential of the image signal supplied through the data line to the pixel to be turned xe2x80x9conxe2x80x9d state is lower than, or is equal to, the potential of the opposite electrode. In this structure, the amplitude of the image signal can also be reduced to reduce the driving voltage in the display apparatus while keeping the second TFT in the xe2x80x9conxe2x80x9d state.
In accordance with the present invention, if the second TFT is of an N channel type, it is preferable that the potential of the image signal supplied through the data line to the pixel to be xe2x80x9coffxe2x80x9d state is higher than, or is equal to, the potential of the common power supply line. That is, when the pixel is turned xe2x80x9coffxe2x80x9d, the gate voltage (the image signal) is not applied enough to turn the second TFT xe2x80x9coffxe2x80x9d completely. The xe2x80x9coffxe2x80x9d state can be realized, combined with non-linear characteristics of the luminescent element. Accordingly, the amplitude of the image signal can be reduced to decrease the driving voltage in the display apparatus and to increase the frequency of the image signal.
In accordance with the present invention, if the second TFT is of a P channel type, conversely to each of the above described structures, a relative relation of each potential is reversed. That is, when the second TFT is of a P channel type, a display apparatus is featured that the potential of common power supply line is set at a higher level than that of the opposite electrode. In this case, it is preferable that the potential of the image signal supplied through the data line to the pixel to be xe2x80x9conxe2x80x9d state is higher than, or is equal to, the potential of the opposite electrode. It is also preferable that the potential of the image signal supplied through the data line to the pixel to be xe2x80x9coffxe2x80x9d state is lower than, or is equal to, the potential of the common power supply line.
In accordance with the present invention, it is preferable that the first TFT and the second TFT are formed by TFTs which are in opposite conduction types. That is, it is preferable that if the first TFT is of an N channel type, the second TFT is of a P channel type, while if the first TFT is of a P channel type, the second TFT is of an N channel type. In this structure, as will be described later in relation to in detail, speeding up of the display operation can be achieved, by only changing the potential of the image signal to turn xe2x80x9conxe2x80x9d to the direction that the resistance of the first TFT at the xe2x80x9conxe2x80x9d state is reduced within the range of the driving voltage in the display apparatus. Since it means that the potential of the image signal to put the pixel xe2x80x9conxe2x80x9d state is changed to the direction that the resistance of the second TFT at the xe2x80x9con statexe2x80x9d is reduced at this time, as a result, a display luminance can be improved. Thus, reduction at the driving voltage and improvement in the display quality can be accomplished simultaneously.
Another embodiment of the invention describes a display apparatus comprising, arranged on a substrate, a plurality of scanning lines, a plurality of data lines intersecting the scanning lines, a plurality of common power supply lines, a plurality of pixels formed by the scanning lines and the data lines in a matrix form, each pixel comprising a first TFT having a first gate electrode which is supplied with a scanning signal through the scanning line, a holding capacitor which holds an image signal supplied through the data line via the first TFT, a second TFT having a second gate electrode which is supplied with the image signal held by the holding capacitor, and a luminescent element comprising an emitting thin film which is provided between a pixel electrode formed by each of the pixels and an opposite electrode opposed to the pixel electrode and emits light due to the driving current which flows between the pixel electrode and the opposite electrode when the pixel electrode is electrically connected to the common power supply line through the second TFT, wherein the first TFT and the second TFT are formed as the thin film transistors which are of opposite conduction types relative to each other.
In accordance with the present invention, since the first TFT and the second TFT are of opposite conduction types, for example, if the first TFT is of an N channel type, the second TFT is of a P channel type, a height of the selecting pulse is to be raised to increase a storage capacity of the first TFT, while the potential of the image signal is to be lowered to reduce the xe2x80x9conxe2x80x9d resistance of the second TFT and to increase an emitting luminance. These optimizations in the scanning signal and the image signal are effective at shifting the gate voltage of the first TFT to the direction of increasing the xe2x80x9conxe2x80x9d current of this TFT in accordance with that of the image signals, which is at the level to turn the luminescent element xe2x80x9conxe2x80x9d state, are written into the holding capacitor, during the selection of the pixel. Therefore, the image signal can be written into the holding capacitor smoothly from the data line through the first TFT. The gate voltage of the first TFT in the event of selecting the pixel corresponds to the potential difference between the scanning signal at the higher potential side and the potential-holding electrode at the time of xe2x80x9conxe2x80x9d (the potential of the image signal to turn xe2x80x9conxe2x80x9d, the potential of the holding capacitor or the gate electrode potential of the second TFT). The gate voltage of the second TFT corresponds to the potential difference between the potential-holding electrode at the time of xe2x80x9conxe2x80x9d and the common power supply line. When using the potential of the potential-holding electrode at the time as a reference, the potential of the scanning signal at the higher potential side and the common power supply line are the same in polarity. If the potential of the potential-holding electrode at the time of xe2x80x9conxe2x80x9d (the potential of the image signal to turn xe2x80x9conxe2x80x9d) is changed, both gate voltages of the first TFT and the second TFT change correspondingly, in the same direction and by the same amount. Therefore, a speed up of the display operation can be accomplished, provided that the image signal potential to turn xe2x80x9conxe2x80x9d is changed to decrease the resistance of the first TFT at the time of xe2x80x9conxe2x80x9d. At this time, since the potential of the image signal to turn xe2x80x9conxe2x80x9d is changed in the direction that the resistance of the first TFT at the time of xe2x80x9conxe2x80x9d is reduced, as a result, a display luminance can be improved. Thus the reduction in the driving voltage and improvement in the display quality can be accomplished simultaneously.
In accordance with the present invention, it is preferable that the gate voltage applied to the second TFT in the pixel at xe2x80x9coffxe2x80x9d state is in the same polarity as of the second TFT in the xe2x80x9conxe2x80x9d state, and also the value of the gate voltage does not exceed the threshold voltage of the second TFT. That is, when the pixel is turned xe2x80x9coffxe2x80x9d, the gate voltage (the image signal) is not applied enough to completely turn the second TFT xe2x80x9conxe2x80x9d state. Thus, the amplitude of the image signal can be reduced to achieve the increased frequency of the image signal.
In this structure, if the first TFT is of an N channel type and the second TFT is of a P channel type, it is preferable that the potentials of the scanning signal to turn the first TFT xe2x80x9conxe2x80x9d and the common power supply line are the same, and the potential of the gate electrode applied to the second TFT of the pixel in the xe2x80x9coffxe2x80x9d state is lower than the potential which is obtained by subtracting the threshold voltage of the first TFT from the scanning signal potential at which the first TFT is turned xe2x80x9conxe2x80x9d. In contrast, if the first TFT is of a P channel type and the second TFT is of an N channel type, it is preferable that the potential of the scanning signal when the first TFT is turned xe2x80x9conxe2x80x9d is the same as that of the common power supply line, and also the potential of the gate electrode applied to the second TFT of the pixel in the xe2x80x9coffxe2x80x9d state is higher than the potential which is obtained by adding the threshold voltage of the first TFT to the scanning signal potential at which the first TFT is turned xe2x80x9conxe2x80x9d. As described above, if the potential of the scanning signal when the first TFT is turned xe2x80x9conxe2x80x9d and that of the common power supply line are equated, the number of levels of each driving signal is reduced. Thus, the number of input terminals to the display apparatus and the number of power sources can be reduced simultaneously, and this results in reduced power consumption.
In accordance with the present invention, it is preferable that one of the electrodes, which is provided at the holding capacitor and is opposite to the electrode to be electrically connected to the second gate electrode of the second TFT, is supplied with a pulse, a potential polarity of which is opposite to the selecting pulse of the scanning signal with a delay behind the selecting pulse. In this structure, since the writing of the image signals into the holding capacitor can be supplemented, the potential of the image signal applied to the gate electrode of the second TFT can be shifted in the direction to increase a luminance, without increasing the amplitude of the image signal.
Further, another embodiment of the invention describes a display apparatus comprising, arranged on a substrate, a plurality of scanning lines, a plurality of data lines intersecting the scanning lines, a plurality of common power supply lines, a plurality of pixels formed by the scanning lines and the data lines in a matrix form, each pixel comprising a first TFT having a first gate electrode which is supplied with a scanning signal through the scanning line, a holding capacitor which holds an image signal supplied through the data line via the first TFT, a second TFT having a second gate electrode which is supplied with the image signal held by the holding capacitor, and an luminescent element comprising an emitting thin film which is provided between a pixel electrode formed by each of the pixels and an opposite electrode opposed to the pixel electrode and emits light due to the driving current which flows between the pixel electrode and the opposite electrode when the pixel electrode is electrically connected to the common power supply line through the second TFT, wherein one of electrodes of the holding capacitor, opposite to that electrically connected to the second gate electrode of the second TFT, is supplied with a pulse, the potential polarity of which is opposite to the selecting pulse of the scanning signal with a delay behind the selecting pulse.
In this structure, since the writing of the image signals into the holding capacitor can be supplemented, the potential of the image signal applied to the gate electrode of the second TFT can be shifted in the direction to increase a luminance, without increasing the amplitude of the image signal.
In any of the aforementioned embodiments, an organic semiconductor film can be used as the emitting thin films, for example.
In accordance with the present invention, in any of the aforementioned embodiments, the second TFT can be formed so as to perform in the saturated region to prevent an abnormal current from generating in the luminescent element, which would result in the generation of a cross-talk, etc. at another pixel because of the voltage drop, or the like.
Further, it is possible to prevent unevenness of the threshold voltage from influencing a display operation by forming the second TFT so as to operating in the linear region.